Searching for Muonphilic Dark Sectors with Proton Beams

Proton beam-dump experiments are a high-intensity source of secondary muons and provide an opportunity to probe muon-specific dark sectors. We adopt a simplified-models framework for an exotic light scalar particle coupling predominantly or exclusively to muons. Equipped with state-of-the-art muon simulations, we compute the sensitivity reach in the parameter space <math display="inline"><mo stretchy="false">(</mo><msub><mi>m</mi><mi>S</mi></msub><mo>,</mo><msub><mi>g</mi><mi>μ</mi></msub><mo stretchy="false">)</mo></math> of the dark mediator, examining in detail the examples of the experiment NA62 in beam-dump mode and the proposed experiment SHiP. We find a significant yield of such exotics in the sub-GeV mass range. Our projections are competitive with those of primary muon-beam experiments and complementary to current constraints, spanning uncharted parameter space and accessing new physics potentially responsible for the <math display="inline"><mo stretchy="false">(</mo><mi>g</mi><mo>-</mo><mn>2</mn><msub><mo stretchy="false">)</mo><mi>μ</mi></msub></math> anomaly.